In the manufacture of wood material panels, in particular wood fiber panels, a sifter or particle-separating apparatus has a housing having a material inlet, at least one air inlet below the material inlet, an exhaust-air outlet above the air inlet, and a coarse-particle outlet below the air inlet. An upper front wall of the sifter housing above the air inlet is oriented at least partially, i.e. over a certain vertical portion, at an acute angle to the vertical.
This type of sifter is used for cleaning particle streams in the wood-materials industry, and in particular for separating undesirable components from a particle-carrying stream. The intent is thus to remove bits of metal, coarse fibers, rust flecks, adhesive particles or clumps, for example, in order to protect downstream facilities or parts thereof, in particular the steel belts of continuously operating wood material presses, from damage. The sifter is particularly preferably used during the manufacture of wood fiber panels for separating coarse particles from the fiber stream, and thus, from the (glued) wood fibers (rubberwood fibers, for example).
Fiber panels here are medium density fiber (MDF) panels, for example. During manufacture of the fibers for wood fiber panels, the wood is initially separated into fibers in a refiner and wet-glued in a blowpipe, for example, and subsequently dried. The sifter is preferably downstream from these components, and particularly preferably downstream from the dryer of such a facility.
The sifter operates as an air sifter in that the material to be classified is introduced into the housing via the material inlet and laterally acted on by an air stream that is blown into the housing via the air inlet. The fibers are entrained by the air stream and discharged together with the air stream via the (upper) exhaust-air outlet and an exhaust air line connected thereto. Coarse particles having a fairly high weight are not entrained by the air stream and fall downward into the area of the coarse-particle outlet that may be provided with a gate so that the (undesirable) coarse particles can be discharged.
A sifter of the described type is known for example from EP 0 795 359. This sifter has a first (upper) material inlet for supplying an upper particle-carrying air stream, and a lower (lower) material inlet therebelow for supplying a lower air stream. The upper air passes into the chamber of the sifter via an upper line, and at the opening of the upper air line the particles are entrained by the air stream of the upper air and swirled upward. A high material concentration is present at the upper edge of the inflow cross section of the upper air, as a result of which it may be difficult for the air stream to entrain the fibers at these locations, in particular for large material quantities. Although even greater material quantities can be managed by the air stream by increasing the velocity of the inflowing air, which has an adverse effect on the classifying. To avoid these disadvantages, horizontal parallel fittings in the form of distribution pipes are provided at the opening of the upper air line into the sifter. The intent is for these distribution pipes to increase the vertical component of the velocity vector of the inflowing air, and for the distribution pipes to prevent material from passing into the line of the upper air and be deposited there. The intent is also to increase the classifying efficiency by additionally supplying lower air via the lower air line. Such a sifter having an upper air line and a lower air line has basically proven satisfactory in practice. However, the known embodiment is capable of refinement. This the concern for the instant invention.
Furthermore, a sifter is known from EP 1 900 445 for separating coarse and fine particles during the manufacture of wood fiber panels, in which multiple inlet openings for the classifying air, one above the other, are likewise provided. These inlet openings for the classifying air are offset in steps in order to improve the cross-flow separation in the conveying direction of the material, i.e. in the flow direction of the stream of classifying air toward the discharge opening. Three inlet openings for the classifying air, one above the other, are preferred.
Finally, U.S. Pat. No. 5,725,102 describes a sifter having “zigzag plates” and operates by both gravimetric and centrifugal action. A zigzag-shaped classifying area is delimited by a deflection line having a downstream material sorting gate so that a division into a fiber-air mixture on the one hand and air on the other hand takes place due to centrifugal forces.